1. Field of the Invention
The present invention relates to a cap of a fuel tank with a pressure control valve for controlling pressure in the fuel tank.
2. Description of the Prior Art
One known example of fuel tank caps is disclosed in JAPANESE UTILITY PATENT PUBLICATION GAZETTE No. 6-88606. FIG. 38 is a sectional view illustrating a fuel tank cap 300. As shown in FIG. 38, the fuel tank cap 300 includes a plastic casing body 302 screwed to a filler neck FN of a fuel tank (not shown), a cover member 330 attached to the casing body 302, and a negative pressure valve 340 received in a valve chamber 304 of the casing body 302 for controlling pressure in the fuel tank. The negative pressure valve 340 includes a rubber valve body 342, a valve supporting member 346 with a fitting aperture 346a for supporting the valve body 342, and a spring 348 for pressing the valve body 342. When the differential pressure between the tank pressure and the atmospheric pressure applied to the valve body 342 increases to or above a predetermined level, the negative pressure valve 340 opens to make the tank pressure approach atmospheric pressure.
In the negative pressure valve 340, when the pressure difference between the tank pressure and the atmospheric pressure becomes equal to or greater than the predetermined level, the valve body 342 temporarily opens to make the tank pressure approach atmospheric pressure. When the tank pressure becomes sufficiently close to the atmospheric pressure, the valve body 342 is closed to cause its lip element 342a to be seated on a seat member 306. In this state, the valve body 342 is deflected to eliminate a gap between the lip element 342a and the seat member 306 and enhance the sealing property.
It is desirable that the negative pressure valve 340 opens quickly to cancel the pressure difference when the pressure difference increases to or above the predetermined level. The circumferential portion of the valve body 342 is, however, bent and seated on the seat member 306. This extends the sealing point and may interfere with a smooth valve-opening action.